I can't post the full link yet, but found an article on confectionerynews dot com - it's currently on their front page for today, but you can also find the article by searching "toxin increase in heated HFCS" on their site. Here's an excerpt:

The formation of a toxic substance when high fructose corn syrup is heated raises concerns for bee keepers, say researchers, and will help inform advice on safe storage of the ingredient for use in human food.

High fructose corn syrup (HFCS) is a sweetener commonly used in food products. HFCS 42, containing 42 per cent fructose, is mostly used in bakery and confectionery products, which HFCS 55 (a mixture of HFCS 42 and 90) is commonly used in soft drinks.

It is also used in bee keeping to stimulate brood rearing in the spring and to increase honey production. Bees’ predilection for the sweetener was first observed when they were seen to swarm to spillages of the syrup around facilities and rail roads.

However the researcher from the USDA report in a new study that the heating of HFCS raises levels of hydroxymethylfurfural (HMF), a toxin that causes gut ulceration and dysentery-like symptoms in bees. In humans it has been linked to DNA damage, and its daughter metabolites levulinic and formic acids have also been seen to cause harm.

"The data are important for commercial beekeepers, for manufacturers of HFCS, and for purposes of food storage. Because HFCS is incorporated as a sweetener in many processed foods, the data from this study are important for human health as well,” they wrote in the Journal of Agriculture and Food Chemistry.